1. Field of the Invention
The present invention relates in general to a method of joining aluminum alloy members and a joined product for press forming. More particularly, the invention relates to such a method of advantageously joining aluminum alloy members made of a 6000 series aluminum alloy (Al—Mg—Si series alloy) with T4 temper and a joined product for press forming obtained utilizing such a method.
2. Discussion of Related Art
Conventionally, so-called aluminum alloy members such as plate members, extruded members and the like made of various kinds of aluminum alloys are subjected to press forming so as to produce various press products such as articles for daily use, machinery parts, building materials, etc. In recent years, products required to exhibit excellent strength such as inner and outer members of body panels of automotive vehicles and wheels are obtained by press forming of the aluminum alloy members. As aluminum alloy members that give such press products, there are widely used materials formed of 6000 series aluminum alloys which have been subjected to a T4 treatment having both of a high degree of strength and excellent workability.
As well known, the 6000 series aluminum alloy is inferior in welding property, so that a joined product obtained by joining, according to a fusion welding method, aluminum alloy members formed of the 6000 series aluminum alloy with T4 temper inevitably suffers from a decrease in its bonding or joining strength
In view of the above, for increasing the bonding strength of the joined product to be obtained by the fusion welding of the 6000 series aluminum alloy members, various techniques are proposed in JP-A-5-117826, JP-A-8-246116 and JP-A-11-199994, for instance. In the disclosed techniques, a joined product obtained by joining the 6000 series aluminum alloy members according to the fusion welding method is subjected to a heat treatment at a predetermined temperature for a suitable time or an artificial aging treatment at a heating temperature within a predetermined range, thereby increasing strength at a welded region. Even if such techniques are employed, it is difficult to avoid occurrence of bonding defects, at the welded region, such as blow holes and involvement of slag which are peculiar to the fusion welding.
On the other hand, an increasing attention has been recently drawn to a friction stir welding process that is a sort of solid-phase welding or joining process in which material members are joined together in a solid-phase without being molten by heat. The friction stir welding process is employed for joining various aluminum alloy members. In view of the problem in the fusion welding described above, various techniques have been proposed in JP-A-11-104860, JP-A-2000-61663, JP-A-2002-346770 and JP-A-2002-294381, for instance. In the disclosed techniques, the 6000 series aluminum alloy members are joined together by the friction stir welding process to form a joined product and the obtained joined product is subjected to an artificial aging treatment at various temperature values for the purpose of improving the bonding or joining strength of the joined product.
As a result of various studies made by the inventors of the present invention on the characteristic of the joined product obtained according to the proposed techniques, i.e., the joined product obtained first by joining the 6000 series aluminum alloy members according to the friction stir welding process and then performing the artificial aging treatment thereof, it was realized that the welded region (stir zone) of the joined product was hardened by the artificial aging treatment, so that the bonding strength of the joined product was improved. It was, however, turned out the following: Heat-affected zones respectively located within considerably narrow areas at boundary portions between the stir zone and respective base material portions have hardness lower than those of the stir zone and the base material portions, so that a stress concentrates on the heat-affected zones when the joined body is subjected to press forming. Consequently, the heat-affected zones tend to easily fracture or break, leading to insufficient elongation of the joined product as a whole.
One (JP-A-2002-294381) of the aforementioned documents discloses the following technique: Among the 6000 series aluminum alloy members, excess Si-content type 6000 series aluminum alloy members are joined by the friction stir welding process to provide a joined product. The obtained joined product is subjected to the artificial aging treatment at a temperature of not higher than 180° C., so that joint strength of the stir zone is restored by 70% or larger with respect to strength of the base material while joint elongation of the stir zone is restored by 50% or larger with respect to elongation of the base material. As a consequence, it is possible to avoid occurrence of cracking at the stir zone and the heat-affected zones in press forming of the joined product, thereby enhancing press formability of the joined product.
According to the studies made by the inventors, however, where such a technique is employed, the hardness of the base material portions is increased, by the artificial aging treatment on the joined product, by an amount corresponding to the increase in the hardness of the stir zone and the heat-affected zone, undesirably deteriorating excellent workability characteristic to the 6000 aluminum alloy. Accordingly, the amount of elongation by deformation of the base material in press forming of the joined product is inevitably decreased, so that the proposed technique does not assure a satisfactory effect of improving the press formability of the joined product as a whole.